Rabies is a zoonotic viral disease which infects domestic and wild animals. Once symptoms of the disease develop, rabies is fatal to both animals and humans. However, when individuals are vaccinated with a rabies vaccine either prior to the virus exposure or after the exposure when combined with thorough cleaning of the wound with antiseptic and anti-rabies antibodies, the individuals are generally well protected. Human rabies vaccines are made from inactivated or attenuated rabies virus and have gone through successive improvements since the time of Pasteur. The first rabies vaccine was developed by Pasteur which was nerve tissue based and virus was inactivated by drying but this vaccine had a risk of activation of the virus and allergic reaction due to the presence of nerve tissue or myelin. Myelin free vaccines prepared from neonatal mouse brains were introduced by Fuenzalida et. al. (Vaccines: Fourth edition, chapter 37, Plotkin, Rupprecht and Koprowski).
Subsequently, Duck Embryo Vaccine (DEV) for rabies was developed. DEV for rabies was prepared from virus propagated in embryonated duck eggs. It was less immunogenic than the brain tissue vaccine. Fourteen to twenty three daily inoculations were recommended for DEV and sometime such high dosages also did not protect against rabies after severe exposure. (Vaccines: Fourth edition, chapter 37, Page number 1018, Plotkin, Rupprecht and Koprowski). The other drawback associated with DEV was that it also had myelin based proteins, which caused side reactions so later on it was banned by the World Health Organization (WHO). Thus there was a long need for highly immunogenic rabies vaccine that could be used safely and effectively at low doses, both for primary immunization and for treatment after exposure. These vaccines would also greatly reduce the number and severity of post vaccinal reactions.
Such a need was satisfied by the development of tissue/cell culture vaccines. The cell culture vaccine is not only safer compared to the former brain tissue vaccines by virtue of the absence of neuronal tissue but also is more efficacious. Several cell culture based vaccines have been developed in order to achieve high immunogenicity and safety like Purified Duck Embryo Vaccine (PDEV), 1st generation vaccines like Human Diploid Cell Vaccine (Wiktor et al., 1964. J. Immunol. 93:353-366) and 2nd generation vaccines like Purified Chick Embryo Cell Vaccine (PCECV), Purified Vero Cell Rabies Vaccine, Rabies Vaccine Adsorbed (RVA), and Primary Hamster Kidney Cell Vaccine (PHKCV) etc. (Ref: JIACM 2006; 7(1): 39-46).
The technical advancement leading to the development of the above vaccines included the adaptation of Pitman Moore strain of rabies virus to continuous cell lines such as Vero cells (e.g. Purified Vero Cell Rabies Vaccine—Abhayrab™ & Verorab™) and MRC-5 human diploid cell culture line [e.g. Human Diploid Cell Vaccine (HDCV)—MIRV-HDC in India] [Ref: JIACM 2006; 7(1): 39-46] or in Duck Embryos in situ (e.g. Purified Duck Embryo Vaccine-PDEV—Lyssavac N) (Ref: Laboratory Techniques in Rabies; Fourth Edition, Edi. by F. X. Meslin, M M Kaplan & H Koprowski, WHO Geneva-1996).
Both Vero and MRC-5 cell lines are continuous cell lines, and hence necessitate the testing of cellular residual DNA in the finished product (Ref. European Pharmacopoeia, 2004) which may be due to risk of either transmission of latent viruses & other agents. Moreover, the yields obtained with Vero cells are substantially low even when PM strain is used for preparing the vaccine. PDEV is a suspension vaccine and hence this vaccine does not qualify for Intra-dermal (ID) application. Moreover, the technology suffers from attaining low yields (1.8-2.2 doses/egg). The process time is also of 88 days which is too long. The commercially available PDEV vaccine uses a preservative thiomersal, which has been linked to possible Autism in young children (ncirs usyd edu au/facts/f thiomersal) Additionally, the process for production of PDEV is long, cumbersome and not preferred for large scale production because it gives low yield. HDCV is considered gold standards vaccine but it is highly expensive. Therefore, there is a need to provide an improved and highly immunogenic rabies vaccine which provides better yield as well as is less expensive using cell culture based technology.
U.S. Pat. No. 4,115,195 (Rudolph Barth et al.) describes a process to manufacture rabies vaccine. It teaches that Chick Embryo Fibroblast cells, along with other cultures of cell strains can be used to make rabies vaccine with various viruses like viruses of strain VP 11, strain Pasteur, PM strain, or homogenized Chick-Embryo material containing viruses of strain Flury LEP (Low egg passage) or Flury HEP (High egg passage). The patent also specifically provides examples for use of rabies virus fixed strain VP 11, Flury HEP and Flury LEP to infect Chick Embryo Fibroblast cells. However, this document does not teach the adaptation of Pitman Moore strain (Wistar strain PM-HDCS, 1503-3M) either to Primary Duck Embryo Fibroblast Cells or to Primary Chick Embryo Fibroblast cells. Also, the media used in the present invention is a unique combination medium, which is not taught in U.S. Pat. No. 4,115,195, and is exclusively designed for PM rabies virus to infect Primary Chick Embryo Fibroblast Cells.
The present inventors have surprisingly found that the Pitman Moore strain could be adapted to Primary chick fibroblast cell culture. Such adaptation has provides a method of preparing the rabies vaccine in large quantities, having excellent yield, with low throughput time and easily scalable. The vaccine produced will be suitable for Intra Dermal application in addition to intramuscular (IM) application, since the vaccine is not a suspension.
According to the present invention, the rabies vaccine prepared by adaptation of Pitman Moore to primary chick fibroblast cell culture is more advantageous than many other continuous cell lines based rabies vaccine and is more readily scalable to large scale commercial vaccine production. The vaccine produced by the present process has a very high yield, efficacy, safety as well as the process is much cost effective than many of the other processes known for preparation of rabies vaccine, preferably when the process is carried out using Polyethylene Terephthalate (PET) Tissue Culture treated (TC) Roller bottles.
The present inventors have surprisingly found that Pitman Moore virus can be severely infected in Primary chick fibroblast cell culture with unique combination medium as described elsewhere, under suitable process conditions as hereinafter described in details, using PET TC Roller bottles. Such preferred embodiments provide vaccine with high yield, greater potency and immunogenicity which makes the vaccine comparatively cost effective.